The invention pertains to a column for preparative liquid chromatography.
Rapid development of the analytical high-pressure liquid chromatography leading to columns with high separation efficiences increased the interest in the preparative columns with similar properties. The reason is that the use of smaller particles of a sorbent enables, similarly as in the analytical columns, to increase the efficiency of columns substantially (as much as by an order of magnitude), provided the sorbent is sufficiently rigid in order to withstand the required pressure gradient. Among basic problems which need to be solved in designing the preparative column there are the distribution of mobile phase and injected sample into the column head and the outlet of sample from the column which have to be arranged in a way preventing from an additional spreading of the sample zone and thus from the decrease in efficiency. A considerable spreading of the injected sample occurs always if a void of space is formed in the column head where the sample is mixed with mobile phase; rinsing of this void leads to rather asymmetric zones of sample. The void may arise, for example, by squeezing the sorbent as a consequence of pressure gradient in chromatography or by a slow dissolution of sorbent particles in mobile phase. All given problems become the more evident the larger is the diameter of column. The inlet and outlet of mobile phase and sample are mostly realized as a central inlet and outlet of the column. They are described columns with a conical shape of the inlet and outlet parts and also columns with complex distributors in the inlet and outlet parts which should secure an uniform distribution and collection of a sample from the whole cross-section of column (for example, Coq B., Cretier G., Rocca J. L.; Journal of Chromatography 186, 457 (1979)). The solutions with the inlet of sample into the column head reliably work provided that a void of space is not formed in the column head. The squeezing of sorbent bed, above all in columns with a large diameter, occurs even with such rigid particles as is the microparticular silica gel. Packing procedures commonly used for analytical columns, which secure the enhanced stability of sorbent bed, i.e. the packing with a suspension of particles at the pressure up to 50-60 MPa, can be applied for preparative columns with difficulty. Thus, formation of such pressure gradient in a column with diameter of 100 mm would require a very complicated packing equipment, a pump in particular. Because operation pressures in the column are lower by an order of magnitude than the above pressure, at least ten times higher pressure resistance of the preparative column would be required in comparison to operation conditions. The tendency is therefore to pack the columns with large diameters by a simple sedimentation in gravitation field. This can be easily achieved, but without a sufficient guaranty that the packing of particles is stable enough under the pressure gradient during operation of the column and the risk of formation of voids in the column head is further increased. Three solutions of this problem are known. The first solution are columns with a sliding piston; the column head, namely its lower part, is formed by a sliding piston which may be slided in such a way that it always presses on the column of sorbent and, at the same time, introduces the sample into sorbent or leads the sample from the sorbent using a suitable distributor, if need be, e.g. in the German Patent Application (FRG) No. 3,021,366 or European Patent Application No. 0040,663. An disadvantage is here the complexity of piston as such and high requirements on the quality of tubes for columns, because the piston must be perfectly sealed against the outer overpressure. Besides, a considerable force has to be developed for shifting the piston in order to compensate the inner overpressure and this force increases with the incrasing diameter. The second posibility consists in manufacturing the column from an elastic material and stabilizing the sorbent column by a radial compression in such a way that a void of space cannot occur (e.g. German Patent Application (FRG) NO. 3,000,475). This solution requires a relatively complex equipment compressing the elastic column and does not completely solve the problem with dissolution of the sorbent and formation of voids during column operation, which problem rapidly grows with the increasing diameter of column, and therefore it is used only for columns with a smaller diameter. The third solution is injection of a sample directly into the sorbent bed on the column head either axially (Katz E., Scott R. P. W.; J. Chromatography 246, 191 (1982)) or from the side of column (Molnar I., Huhn A., Lamer W.; International Laboratory 14, no. 3, 10 (1984)). Equipments of this type are again complicated and sensitive to handling and the sample inlets become often choked. In addition, they always need to use two pumps, one of which leads a mobile phase to the column head while the other leads the sample and mobile phase into the sorbent. If a high efficiency wants to be achieved, the proper setting of both flow rates makes a great difference. The described inlets of sample into the sorbent have been used only in analytical columns. They can be obviously used also for columns with larger diameters, but their complexity would further increase.